The rotating shafts of pumps and other fluid-handling devices have for many years been sealed with mechanical packings consisting of preformed rings or sections of rope-like packing materials. During the course of normal operations these various packing materials become worn and must be replaced in order to maintain an acceptable seal in the equipment.
Normal maintenance practices for replacement of shaft packings can involve significant manpower and material wastage with the attendant costs, because most fluid-handling facilities (chemical process plants, oil refineries, pipelines, pumping stations, factories, etc.) use a wide variety of sizes of pumps and other fluid-handling devices. Since each different size of pump shaft and stuffing box requires a different size of packing, two alternative and potentially wasteful replacement methods have come into general use.
In the first method, split packing rings of the exact circumference required for a given pump shaft are used for replacement of the factory-installed rings. This method has the substantial disadvantage of requiring an inventory of a large number of sizes of packing rings. Further, in many cases a maintenance man will service a number of different pumps each working shift. Since he cannot normally carry with him a supply of all the different exact size packing rings which may be required during his shift, he must make numerous trips to a supply truck or warehouse to obtain the exact size rings needed for the particular job. These trips result in a large amount of nonproductive and costly man-hours.
The alternative method requires the workman to carry with him several coils of continuous rope-like packing material, the different packing ropes being of different cross-sectional sizes. At each pump, the workman cuts off a length of the rope which is supposed to be equivalent to the length of the circumference of the shaft to be sealed (see FIG. 1). In actual practice, however, the length of rope cut by the workman is rarely of the exact length needed, but rather is normally too short or too long. If the length cut is too short, the entire length is discarded and a new length cut. Often a workman tries to err on the side of convenience and thus cuts a length considerably longer than that needed, thereafter cutting off the excess. In either case, however, wastage of the product is common. In addition, rings cut from rope have a decided tendency to "keystone" or spread apart at their outer edges. Preformed rings do not do this.